BEL constant in MAINDEC-8E-D0AB-PB

[Vincent Slyngstad]

From: Pontus Pihlgren: Wednesday, August 17, 2016 11:26 PM
> I've been running and looking at the PDP-8/E INSTRUCTION TEST 1. And it 
> is supposed to write the BEL (07) character after each round of passed 
> tests. However the code loads and outputs the constant "0207", which is 
> 07 with the eigth bit set. I think the 8/E serial interface outputs all 
> eight bits in this case?

Yes; the serial interfaces were also used to send and receive 8-bit binary 
as well as ASCII, so 8 bits are sent and received.

> I suppose a real ASR-33 would ignore the eight bit?
> 
> Why does the code not load 07?

The teletypes used were typically set for "mark parity", so the ASCII 
input data had the high bit set.  The program is presumably mimicking 
the code received from the TTY if BEL were input.  The TTY will indeed 
set the bit for keyboard input (paper tape in/out is 8 bits), and ignore it 
for teleprinter output.

The character set itself is 7 bit, and it is conventional not to treat the 
eighth bit as part of the character, so the ASCII charts pretend it doesn't 
exist.

    Vince